Soft contact lenses can be divided into two basic types, water absorptive and non-water absorptive. The water absorptive lenses, commonly referred to as "hydrogel lenses", are generally prepared from 2-hydroxyethyl methacrylate (HEMA) or copolymers with HEMA as the major component. High water content lenses have also been produced from vinylpyrrolidone. Non-water absorptive soft contact lenses are produced from silicone rubber or like materials. Such soft contact lenses can have one or more of the following disadvantages: poor durability, less visual acuity than hard lenses, poor oxygen permeability, and/or ease of bacterial contamination.
Hard contact lenses produced from polymethyl methacrylate (PMMA) have been known in the art for many years and offer the advantages of optical clarity, dimensional stability and durability. Although PMMA has been the standard of the hard contact lens industry, it has at least two drawbacks. Because PMMA is marginally hydrophilic a lens wearer may experience discomfort as a result of a foreign body reaction. Secondly, oxygen gas transport through PMMA contact lenses is extremely low which dictates that the lenses cannot be worn continuously for an extended period of time. Since the cornea receives its oxygen directly from the atmosphere the PMMA lens wearer often experiences corneal swelling and irritation due to prolonged oxygen deprivation.
Within the past ten years commercial cellulose acetate butyrate (CAB) has been utilized in an attempt to provide a hard contact lens that will transport oxygen. Although CAB exhibits modest oxygen permeability it lacks other essential qualities necessary for a contact lens material. The scratch or mar resistance of CAB contact lenses is poor which may be a reflection of the relative softness of CAB when compared to PMMA. Additionally, CAB lenses are often dimensionally unstable.
More recently, siloxane containing copolymers have been introduced as oxygen gas permeable hard contact lens materials. These polymeric compositions are generally prepared by copolymerizing methyl methacrylate with a siloxanyl alkyl ester of methacrylic acid. Contact lenses containing substantial amounts of organosiloxane groups tend to be hydrophobic. Attempts to impart hydrophilic properties to such systems include the incorporation of a wetting agent and treatment of the lens surfaces. Incorporation of a wetting agent can improve the wettability of the lens but may also render the lens translucent when used in excessive amounts. Contact lenses containing such wetting agents can be tolerated by the wearer but tend to accumulate proteinaceous matter from the tear fluid. This results in decreased transparency of the lens and wearer discomfort. Surface treatment of the lens affords a wettable surface but can lack permanence. Any scratches or adjustments made on such lenses exposes the hydrophobic bulk material. Repeated surface treatment is then necessary which can be inconvenient.
The contact lens field has long known the advantages of silicone polymers for use in contact lenses. Poly(dimethylsiloxane) polymers are transparent and highly permeable to oxygen, although use of the these polymers in contact lenses can present difficulties in the fabrication and finishing of lenses because of the rubbery nature of the polymers. Contact lenses produced from poly(dimethylsiloxane) are often inherently hydrophobic and often must be surface treated to render the surfaces wettable by tears.
It's known that the use of a methacrylate monomer containing a silicone moiety can be copolymerized with the standard monomer utilized in conventional hard contact lenses, i.e., methyl methacrylate, to obtain a copolymer of varying hardness values depending upon the ratio of hard and soft monomers employed. Thus, some attempts have been made in the art to produce hard oxygen-permeable contact lenses. For example, U.S. Pat. No. 3,808,178 discloses a copolymer of methyl methacrylate with a siloxanyl alkyl ester of methacrylic acid. The use of special wetting agents and cross-linking agents are also taught in U.S. Pat. No. 3,808,178.
In U.S. Pat. No. 4,152,508 the use of an itaconate ester copolymerized with a siloxanyl alkyl ester of methacrylic acid is disclosed. The siloxanyl alkyl ester provides for high permeability and the itaconate ester gives increased rigidity, hardness and some degree of wettability. In addition, specific cross-linking agents and hydrophilic monomers are incorporated which provide dimensional stability and wettability to contact lenses generated therefrom.
The compositions disclosed in U.S. Pat. Nos. 4,216,303 and 4,242,483 are branched siloxanyl alkyl esters of methacrylic acid essentially as suggested by the prior patents.
U.S. Pat. No. 4,153,641 and 4,189,546 teach the use of monomeric polysiloxanes end capped with activated, unsaturated groups. By varying the type and amount of comonomer as well as the moiety both hard and soft polymeric compositions are said to be possible. .alpha., .omega. Bis (4-methacryloxybutyl) polydimethylsiloxane is disclosed in which the poly(organosiloxane) moiety varies from about 0 to 800 units in length. For a hard contact lens the poly(organosiloxane) moiety should be of a rather short length, perhaps 0 to 10 units long, to avoid incompatibility in the final composition due to phase separation. Therefore, when comparing the monomeric polysiloxanes disclosed in both U.S. Pat. Nos. 4,153,641 and 4,189,546 with siloxanyl alkyl esters of methacrylic acid disclosed in U.S. Pat. Nos. 3,808,178 and 4,152,508, on the basis of utility in gas permeable hard contact lens compositions, similarities are noted. Whereas the siloxanyl alkyl esters of methacrylic acid disclosed in U.S. Pat. Nos. 3,808,178 and 4,152,508 are monomers, that is, contain one polymerizable unsaturated group, the monomeric polysiloxanes disclosed in U.S. Pat. Nos. 4,153,641 and 4,189,546 contain two such polymerizable unsaturated groups.